The winter solstice (December 21/22) makes the most sense to me. It's only dependent on the Earth's position on its orbit and its tilt, so it should be very predictable and stable, and is easily observable from having the shortest day of the year on that day. (Of course, in the southern hemisphere they get the longest day of the year.)

Off the poll options, the southward equinox comes closest, being a quarter of an orbit off.